The organic luminescence devices utilizing organic substances have favorable prospects for the use as low-priced large-size full color display device of solid luminophor type, and have been developed in large numbers. In general, an organic luminescent device is constituted of a luminescent layer and a pair of facing electrodes between which the luminescent layer is sandwiched. The luminescence is a phenomenon that, when an electric field is impressed between both electrodes, the negative electrode emits electrons and the positive electrode emits positive holes and these electrons and positive holes are injected into a luminescent layer and recombined therein to return the energy level to the valence electron band from the conduction band, thereby liberating the energy as light.
Although conventional organic luminescent devices require high driving voltage and have low luminance and low luminous efficiency, the organic EL device comprising a lamination of thin films containing organic compounds which luminesce with high fluorescent quantum efficiency when a low voltage of 10 V or below is impressed thereon has recently been proposed (Applied Physics Letters, 51, p.913 (1987)) and much attention has been focused thereon. More specifically, such a device produces green luminescence of high luminance by using a metallic chelate complex in the electron-transporting layer, a fluorescent substance in the luminescent layer and an amine compound in a positive hole-transporting layer. Further, when the utilization of organic luminescent devices as full color display or light source is intended, the devices are required to give the three primary colors or white color from a practical point of view. Such being the case, the devices doped with fluorescent dyes to emit light of a desired color were reported (Journal of Applied Physics, vol. 65, p. 3610 (1989)). The technique adopted therein is effective particularly in the case of red luminescence where the density quenching is so serious that it is difficult to form a luminous layer with a fluorescent dye alone, and satisfactory color purity and high luminance are achieved thereby. However, it bears a problem such that, in producing the device doped with a dye by the use of an evaporation technique, a host material and a slight amount of fluorescent dye are co-evaporated; as a result, the operation is complicated and the devices produced tend to be uneven in performance. From the viewpoint of simplifying the production process and ensuring even performance for the products, therefore, it has been desired to develop luminescent materials having satisfactory color purity and enabling the formation of a luminescent layer with a dye alone, especially a red luminescent material which can achieve satisfactory chromaticity and high luminance even when it form a luminescent layer by itself.
On the other hand, although the organic luminescent devices which are successful in achieving high-luminance luminescence are devices comprising organic substances laminated by vacuum evaporation, the production of devices in accordance with a coating method is advantageous from the viewpoints of simplicity of the production process, processability and easiness of size expansion. However, the devices produced by conventional coating methods are inferior to the devices produced by evaporation methods in luminance and luminous efficiency, and so the big problem which confronts them is to achieve highly efficient luminescence having high luminance. Further, the device comprising a coating of the organic low molecular compound dispersed in an organic polymer medium has a problem that, when the device is made to luminesce for a long time, the planar luminescence of even quality becomes hard to obtain due to aggregation of the organic low molecular weight compound.
In recent years, various substances having fluorescence have also been used for filter dyes, color conversion filters, dyes for a photographic material, sensitizing dyes, pulp coloring dyes, laser dyes, fluorescent drugs for medical diagnosis and organic luminescent device materials, and the demands therefor have been growing. However, there are known only few red fluorescent dyes having strong fluorescence intensity and high color purity, and so the development of new materials has been desired.